1. Field of the Invention
The present invention relates to a magnetic recording medium used in a magnetic disk apparatus and the like, a method for manufacturing the magnetic recording medium, and a magnetic recording/reproducing apparatus using the magnetic recording medium.
2. Background Art
In recent years, the recording density of magnetic recording media has been improved as the capacity of magnetic recording/reproducing apparatuses has increased. In the last few years, there has been commercialized a magnetic recording/reproducing apparatus that employs a perpendicular magnetic recording method in which the magnetized state, which could be problematic in high-density magnetic recording, is substantially insensitive to thermal unstableness (superparamagnetic effect) to achieve a recording density of 100 to 200 Gb/in2. To achieve a higher recording density of 200 Gb/in2 or higher, the recording medium itself needs improvement. The current recording medium is called a medium which has a continuous magnetic recording layer, in which each layer that forms the medium is formed by sputtering on the entire substrate in a uniform manner. When the recording density is 200 Gb/in2 or higher, a fringe magnetic field leaked from the side surfaces of the magnetic recording head frequently causes data to be written to adjacent data tracks, resulting in degradation of recorded information in the magnetic form. Furthermore, when the reproducing head is used to read information in the magnetic form on a data track, leakage flux from adjacent tracks reduces the S/N ratio. To avoid such phenomena and further improve the recording density, there has been proposed a discrete track medium having no magnetic recording layer between a data track having magnetic information and the adjacent data track, as shown in FIG. 1. In FIG. 1, reference numeral 11 denotes a substrate. Reference numeral 12 denotes a soft magnetic underlayer. Reference numeral 13 denotes an intermediate layer. Reference numeral 14 denotes a data track. Reference numeral 15 denotes a groove between data tracks. Reference numeral 16 denotes the cross-track direction.
In an even higher recording density of 500 Gb/in2 to 1 Tb/in2, there has been devised a dot-patterned medium in which not only adjacent recording tracks are magnetically separated but also adjacent recording bits formed in the same recording track are magnetically separated. FIG. 2 shows a dot-patterned medium. In FIG. 2, reference numeral 21 denotes a substrate. Reference numeral 22 denotes a soft magnetic underlayer. Reference numeral 23 denotes an intermediate layer. Reference numeral 24 denotes a recording bit. Reference numeral 25 denotes a groove between recording bits. Reference numeral 26 denotes the cross-track direction.
Unlike the conventional magnetic recording medium having a flat structure, the discrete track medium and the dot-patterned medium are sometimes collectively referred to as a patterned medium meaning that they include a fine pattern having an indented structure. The indented structure herein means, as shown in FIGS. 1 and 2, that a microprocessed indented structure is formed somewhere on the substrate or in the soft magnetic underlayer, the intermediate layer, or the magnetic recording layer stacked on the substrate. Alternatively, the indented structure may be an indented structure followed by planarization. Thus forming such an indented structure causes magnetic separation or non-uniformity between adjacent recording tracks or between recording bits.
The fine pattern having an indented structure includes a concentric or spiral discrete track structure or a dot pattern that magnetically separates adjacent recording bits formed on the same recording track, the discrete track structure and recording bits being used to record and reproduce magnetic information in a data area. The fine pattern desirably also includes a servo pattern in a servo area used in servo control of the recording/reproducing head. The patterned medium can be defined as a medium having a microprocessed indented structure formed either in the data area in which magnetic information is recorded or the servo area in which servo information for the recording/reproducing head is written so as to cause magnetic separation or non-uniformity. Therefore, a medium having a flat data area as in the conventional magnetic recording medium is still regarded as a patterned medium, when the medium has a microprocessed indented structure and hence a magnetically intermittent or non-uniform area in the pattern in the servo area used in servo control of the recording/reproducing head.
Patterned media are broadly classified into two types in terms of their structures, as shown in FIGS. 3 and 4. FIG. 3 shows an example of one of the structures, a patterned medium with patterns on a substrate surface in which a fine pattern is directly formed in a substrate 31 and various magnetic films are staked thereon. In FIG. 3, reference numeral 32 denotes a soft magnetic underlayer. Reference numeral 33 denotes an intermediate layer. Reference numeral 34 denotes a magnetic recording layer. Reference numeral 35 denotes a data track. Reference numeral 36 denotes the pitch between data tracks. Other structure has a patterned indented structure in a metallic layer or a nonmetallic layer stacked on the substrate. FIG. 4 shows an example of a patterned medium with patterns on a magnetic recording layer, in which the flat magnetic recording layer formed on the substrate is microprocessed into dots or discrete tracks. In FIG. 4, reference numeral 41 denotes a substrate. Reference numeral 42 denotes a soft magnetic underlayer and an intermediate layer. Reference numeral 43 denotes a magnetic recording layer left after a cutting process (projections corresponding to data tracks or recording bits). Reference numeral 44 denotes an indentation in the cut magnetic recording layer (corresponding to the portion between data tracks or the portion between recording bits). Reference numeral 45 denotes nonmagnetic material filled in an indentation.
As an example of how to fabricate a fine pattern in a patterned medium, IEEE Trans. Magn. Vol. 40, No. 4, 2510 (2004) discloses a method for forming a fine pattern by using a resist pattern (reference numeral 55) fabricated through electron beam lithography as a mask and cutting (56) a magnetic recording layer 54 to form a fine pattern, as shown in FIGS. 5A to 5C. In FIGS. 5A to 5C, reference numeral 51 denotes a substrate. Reference numeral 52 denotes a soft magnetic underlayer. Reference numeral 53 denotes an intermediate layer. Reference numeral 54 denotes the magnetic recording layer. Reference numeral 55 denotes the resist pattern fabricated through electron beam lithography. Reference numeral 56 denotes the cutting process. Reference numeral 57 denotes the magnetic recording layer cut into a patterned indented structure. Reference numeral 58 denotes a data track or a recording bit. The resist pattern indicated by reference numeral 55 in FIGS. 5A and 5B may be formed through a nanoimprinting method in some cases.
IEEE Trans. Magn. Vol. 41, No. 2, 670 (2005) discloses an example of how to fabricate a patterned medium with patterns on a substrate surface. As shown in FIGS. 6A to 6D, the method includes the steps of fabricating a resist pattern 62 on a substrate 61 through electron beam lithography, using the resist pattern 62 as a mask to form a fine pattern 64 in the substrate surface through reactive ion etching indicated by reference numeral 63, and stacking a soft magnetic underlayer 65, an intermediate layer 66, and a magnetic recording layer 67 on the fine pattern 64 in a uniform manner. A patterned medium having the structure shown in FIG. 6D is thus provided. In FIG. 6D, reference numeral 68 denotes a data track or a recording bit.